A powerful cosmic entity, an immortal being representing all current and future life. It has, at times, chosen a host to live within, granting the vessel outstanding power. It has the power to create or destroy. The Phoenix has had various hosts, but the most well known and frequent host is the mortal mutant Jean Grey.

Make that can definitely defeat Galactus. Rachel Summers practically defeated him with only a fragment of the Phoenix Force.

Edit: As has been pointed out to me: Galactus was weak at the time. But the White Phoenix of the Crown is one of the most powerful beings in the known Marvel Universe, it is generally agreed that only the One Above All, and a handful of similarly powerful beings are more powerful than it. Such as the Living Tribunal.

This thread pretty much covers it, although I'm trying to find the feats thread, which goes into greater detail.